1. Field of the Invention
This invention relates to thermoplastic resins having excellent thermal resistance and moldability and suitable for use as molding materials, as well as to processes for preparing such thermoplastic resins. Among the thermoplastic resins of the present invention, those having a specific chemical structure additionally have excellent optical properties and can be used as optical resins.
2. Description of the Prior Art
Polyesters, particularly aromatic polyesters, are being used as so-called engineering plastics because of their excellent thermal resistance and mechanical properties. Vinyl polymers, though being generally inferior in thermal resistance to aromatic polyesters, are also being widely used because of their excellent moldability and mechanical properties. However, from the viewpoint of moldability, aromatic polyesters are disadvantageous in that they have a high molding temperature and poor fluidity because of their high thermal resistance. Moreover, crystalline polyesters have low impact strength, especially when notched, and are generally used in the form reinforced with glass fibers and the like. On the other hand, vinyl polymers as typified by polymethyl methacrylate and polystyrene have excellent moldability, but have low thermal resistance, so that they are distinct from so-called engineering plastics. As described above, these polymers have both merits and demerits. Accordingly, there is a demand for the development of a material having a desirable combination of their merits.
At the same time, polymethyl methacrylate and polycarbonates are being commonly used as optical resins. However, polymethyl methacrylate, though having excellent optical properties, is disadvantageous in that it has low thermal resistance and high water absorption, and in that molded articles formed thereof are subject to deformation. On the other hand, polycarbonates have excellent thermal resistance and resistance to moisture absorption, but are disadvantageous in that they have poor moldability and exhibit a high degree of birefringence due to the orientation produced during molding. For optical resins, it is also desirable to develop a material in which the merits of these polymers are appropriately combined.
As an attempt to improve the moldability of engineering plastics while retaining their thermal resistance and mechanical strength, it has been known to prepare a blend of compatible polymers (e.g., polyphenylene ether and polystyrene). However, since many polymers are generally incompatible with each another, it is practically impossible, with rare exceptions, to improve the properties of various engineering plastics by polymer blending.
Moreover, many attempts have also been made to improve the properties of optical resins. For example, the properties of polymethyl methacrylate have been improved by copolymerization with hydrophobic monomers, and the molecular orientation of polycarbonates has been weakened by a decrease in molecular weight and improvements in injection molding technique. In addition, it is disclosed in U.S. Pat. No. 4,373,065 that birefringence can be offset by blending compatible polymers. However, since this technique is only effective for compatible polymers, its applicability is rather limited.